The present invention relates to a system for converting digital audio or visual data recorded at a sampling frequency to digital data to be sampled at another sampling frequency.
A recordable compact disc (R-CD) is capable of not only reproducing but also recording audio/visual data. The sampling frequency for the R-CD is 44.1 KHz which is the same as the sampling frequency of the CD. On the other hand, a digital-audio tape (DAT) recorder normally uses a 48 KHz sampling frequency, and the broadcast by satellite (BS) system uses 48 KHz or 32 KHz sampling frequency. It is desirable that the digital data recorded on the DAT or transmitted through the BS system can be recorded on the R-CD. In order to perform such a recording, the digital data must be converted into data at the sampling frequency of 44.1 KHz.
Referring to FIG. 3, in a conventional converting system, data which is sampled at a sampling frequency fsin, such as 48 KHz or 32 KHz and quantized is converted into an analog signal by a D/A converter 1. The analog signal is converting to a digital signal by an A/D converter 2 by sampling at a sampling frequency fsout of 44.1 KHz for the R-CD.
However, in the course of the conversion from the digital signal to the analog signal by the D/A converter and vice versa by the A/D converter, the signal-to-noise ratio is aggravated, and furthermore, the signal is distorted. As a result, the output data inevitably becomes erroneous, thereby rendering the quality of the reproduced sound inferior.
In another conventional conversion system shown in FIG. 4, the input data sampled at a frequency fsin is fed to an interpolator 3 having a DSP (dynamic support program) capable of calculation at a high speed, and a multiplier. The data are sampled at a frequency of N.multidot.fsin which is the least common multiple between the sampling frequencies fsin and fsout. The output of the interpolator 3 is applied to a decimator 4 where the output is divided by a value of M, which is a quotient of the least common multiple divided by the frequency fout. In other words, the digital data obtained at N.multidot.fsin is sampled at the sampling frequency fsout.
However, the value of the multiple N.multidot.fsin becomes extremely large, so that the synchronization is difficult. Accordingly, the electronic circuit of the system becomes complicated.